Method for regulating the mixture in an internal combustion engine

ABSTRACT

The invention relates to a method for regulating the mixture in an internal combustion engine by means of a catalytic converter and a lambda probe that is placed downstream of the catalytic converter. Depending on the historical signal values, said method determines whether intervention in the formation of the mixture is required, whether the existing signal is only decreasing slowly, which necessitates slow regulatory intervention, or whether the signal of the probe placed downstream of the catalytic converter decreases rapidly, which necessitates rapid regulatory intervention. The difference types of intervention enable the volume of the catalytic converter to be reduced, thus preventing high consumption in the warm-up phase or the poor start-up behavior of large catalytic converters.

The present invention relates to a method for mixture control in aninternal combustion engine with a catalytic converter and a lambda probedownstream of said catalytic converter.

DE 102 06 399 C1 discloses a method for forced activation of a lambdacontrol system which improves exhaust gas conversion in the case of athree-way catalytic converter, wherein mixture control havingalternately rich and lean exhaust gas packets is performed varyinglyaround a lambda setpoint value. For particularly reliable exhaust gasconversion, so-called fine dosing of the exhaust gas packets isperformed.

To improve exhaust gas conversion still further, efforts are made toreduce the size of the catalytic converter, as although a largecatalytic converter allows good buffering of mixture faults, it requiresa large amount of energy in the heating-up phase and exhibits poorstarting behavior.

The object of the invention is to provide a method of mixture controlwhich reliably ensures high conversion quality even with reduced sizecatalytic converters.

This object is achieved according to the invention by a method havingthe features set forth in claim 1. Advantageous embodiments form thesubject matter of the sub-claims.

With the method according to the invention, a control unit successivelyreads in the lambda values measured by the lambda probe and compares thecurrent lambda value with a previously read-in lambda value. If thecomparison indicates a fall in the lambda value, the control unit caninitiate a mixture change. This mixture change is initiated if thelambda value has fallen by or by more than a predefined constant. Tothis end the change in the lambda value is compared with the constant. Alambda value falling by more than a predefined constant indicates thatcatalytic converter breakdown is imminent, and so direct intervention inthe formation of the mixture takes place via the control unit. On theother hand, if the lambda value falls by less than the predefinedconstant, the control unit initiates a check to ascertain whether thelambda value continues to fall for a number of subsequent measuredvalues. In this checking mode, also known as dynamic mode, interventionin the mixture formation process does not therefore takes placeimmediately. This method allows unnecessary interventions in mixtureformation to be reduced, thereby making it possible for the size of thecatalytic converter to be reduced while at the same time ensuringreliable exhaust gas conversion.

In a preferred embodiment, a reference value is calculated from thecurrent lambda value during checking of the subsequent measured valuesand a mixture change is initiated if firstly more than a minimum numberof measured values have been checked and secondly if the reference valueis less than a predefined constant. Intervention does not thereforeoccur in the event that the reference value is greater than thepredetermined constant or a minimum number of measured values has notyet been checked since the first fall in the lambda signal. The aboveconditions ensure that not every control intervention in mixtureformation is suppressed in checking mode, but that intervention onlyoccurs under particular conditions.

It has also been found advantageous to define a minimum value and amaximum value for the lambda values. These values are preferablydetermined as a function of the operating state, in particular of theair mass flow and/or RPM. The reference value is then obtained as thequotient of the current lambda value minus the minimum value divided bythe difference between the maximum value and minimum value. In thisdefinition, the reference value thus defined can become greater than 1and less than 0. If the values of the current lambda value are greaterthan or equal to the maximum value, the reference value will be greaterthan or equal to 1. If the current lambda value is less than the minimumvalue, the reference value will be negative.

In monitoring mode, intervention in mixture formation preferably occursby changing the frequency and/or amplitude of a forced activation. In apreferred embodiment, intervention in the mixture change is implementedby suppressing the lean exhaust gas packets of the forced activation. Aslight increase in the mean value therefore occurs via the forcedactivation. Therefore, if a slow fall in the lambda value is determinedin monitoring mode, slow intervention in mixture formation takes placeif the reference variable shows corresponding values and a minimum timehas elapsed since the last fall.

In a preferred embodiment, checking of the subsequent measured values isterminated if the lambda values does not continue to fall within apredefined number of measured values. The resetting of dynamic modeensures that signal changes occurring much later are no longerinterpreted against the background of the earlier signal change. In apossible further development of the method according to the invention,the constants, e.g. the constants for the fall in the lambda values, thenumber of measured values to be checked and/or the minimum number ofmeasured values required for initiating intervention in dynamic mode,are determined as a function of the operating point. It is conceivablefor all constants, combinations of constants or only a single constantto be determined on an operating point dependent basis. Operating pointdependence is preferably based on the current exhaust gas composition.

The monitoring duration and the number of lambda values to be monitoredcan be implemented as function of time, specified as a physical timeduration or on a segment-dependent basis in relation to the exhaust gaspackets. It is also possible to make the duration dependent on theoxygen mass balance.

The method according to the invention will now be explained in greaterdetail with reference to the accompanying drawings in which:

FIG. 1 shows a slowly falling lambda signal for which no controlintervention occurs,

FIG. 2 shows a slowly falling lambda signal for which controlintervention occurs via forced activation, and

FIG. 3 shows a heavily falling lambda signal initiating immediatecontrol intervention.

FIG. 1 shows the sequence of the post-cat signals 10 over the number ofsegments. The post-cat sensor is a binary sensor whose signals areanalyzed in the transition range of rich and lean mixture formation. Themeasured post-cat signal VLS_DOWN is set in relation with two operatingpoint dependent maximum and minimum values. The maximum valueVLS_DOWN_MAX and the minimum value VL_DOWN_MIN preferably depend on thecurrent mass air flow (MAF) and the engine speed (N). Using the minimumand maximum value, a reference value FAC_VLS_DOWN is determined. Thereference value is calculated according to the following formula:${{FAC\_ VLS}{\_ DOWN}} = \frac{{VLS\_ DOWN} - {{VLS\_ DOWN}{\_ MIN}}}{{{VLS\_ DOWN}{\_ MAX}} - {{VLS\_ DOWN}{\_ MIN}}}$The reference value assumes values less than 0 when VLS_DOWN is lessthan VLS_DOWN_MIN. If the current lambda value is greater than themaximum value (VLS_(—DOWN>VLS)_DOWN_MAX), values greater than 1 may alsooccur.

In the course of the method it is established whether a falling VLS_DOWNsignal of the post-cat sensor is present. To this end the currentVLS_DOWN value (VLS_DOWN) is compared with the previous VLS_DOWN value(VLS_DOWN_OLD). If the current value has fallen compared to the previouslambda value, the relevant gradient is calculated:VLS _(—) DOWN _(—) GRD=VLS _(—) DOWN _(—) OLD−VLS _(—) DOWN

With the above sign convention, a positive gradient (VLS_DOWN_GRD>0)means that the post-cat sensor-signals are falling. A rising gradienttherefore means an increasing fall in the signal. In order to ascertainwhether an increasing fall in the signal is present, the gradient iscompared with a previous gradient (VLS_DOWN_GRD_OLD). If the gradient isfound to have increased, a flag indicating dynamic mode is set:LV_VLS_DOWN_DYN=TRUE.

As long as the dynamic state is set, the value for the past gradient(VLS_DOWN_GRD_OLD) is only overwritten if a current gradient genuinelygreater than 0 occurs. If a plurality of measured values with constantpost-cat sensor signals (VLS_DOWN_GRD=0) come after the dynamic statehas been set, the past gradient of the post-cat signals is notoverwritten. Only if a rising gradient (VLS_DOWN_GRD>0) occurs is thepast gradient (VLS_DOWN_GRD_OLD) overwritten with a new value for thegradient.

The method according to the invention will now be explained in furtherdetail with reference to the following examples:

After a first detection of a falling post-cat sensor signal VLS_DOWN, acounter is incremented with each segment (CTR VLS_DOWN_CONST). Thecounter is then compared with a predefined constantC_CTR_VLS_DOWN_CONST. If the counter is greater than the constant, thedynamic state LV_VLS_DOWN_DYN is reset and the counterCTR_VLS_DOWN_CONST is re-zeroed. This means that the dynamic state ismaintained for a certain time or a certain number of segments(C_CTR_VLS_DOWN_CONST). If the post-cat sensor signal falls no furtherduring this time, no dynamic state will be present and no controlintervention will occur. A slow fall in the post-cat sensor signalrelative to the constant C_CTR_VLS_DOWN_CONST is not recognized as acritical dynamic and is handled by a function described further below.

FIG. 1 explains the above-described case in greater detail. Intransition from measured value 12 to measured value 14, the post-catsensor signal falls and the counter is incremented. The dynamic bit 16is simultaneously set to 1 (=TRUE) with the transition from 18 to 20. Inthe subsequent segments the counter (CTR_VLS_DOWN_CONST) is incrementedand the dynamic bit 16 is again reset for the transition from 22 to 24if the predefined constant (5 segments in the example shown) isexceeded. As shown in FIG. 1, in the event of a subsequent drop in themeasured post-cat sensor signals 26, 28, 30, no control intervention isinitiated, as the interval between the falling signals is always greaterthan the predetermined duration of five segments.

Now referring to FIGS. 2 and 3, as the result the falling post-catsensor signals 32, 34 in FIG. 2 or 36, 38 in FIG. 3, the dynamic stateis activated. The dynamic indicating bit LV_VLS_DOWN_DYN_DOWN is set to1 in 40 or 42. In dynamic mode the counter CTR_VLS_DOWN_DYN isincremented with each segment. In the example shown in FIG. 2 thepost-cat sensor signal 44 continues to fall. In this case a controlintervention takes place, tending to prevent all lean exhaust gaspackets of the forced activation of the catalytic converter. As alreadyexplained above, in the case of a three-way catalytic converter, a goodconversion rate requires forced activation whereby slightly lean andslightly rich exhaust gas packets are used alternately according to aparticular pattern. Deactivation of the lean packets therefore ensures aricher total mixture averaged over time. Control intervention occurs ifboth the following conditions are met:CTR_VLS_DOWN_DYN>C_CTR_VLS_DYN_THD andFAC_VLS_DOWN<C_FAC_VLS_DOWN_DYN.

The first part of the condition ensures that control intervention onlytakes place if the second falling post-cat sensor signal 44 occurs aftera minimum number of segments after the first fall 34. The minimum numberof segments is denoted as constant C_CTR_VLS_DYN_THD. In addition,control intervention only occurs if the reference value FAC_VLS_DOWN isless than a predefined constant C_FAC_VLS_DOWN_DYN. In the example shownin FIG. 2, the slight fall in the post-cat sensor signal 44 thereforecauses a control intervention which only suppresses the lean exhaust gaspackets of the forced activation and therefore slowly results in richingaveraged over time. By his means it is possible to respond to a slowfall in the post-cat sensor signals by a slow intervention.

The example illustrated in FIG. 3 shows how an initial fall in thepost-cat sensor signal 46 activates dynamic mode 48. With dynamic modeactivated, in the example in FIG. 3 the post-cat sensor signal 50continues to falls. If this fall fulfills the condition:VLS_DOWN_GRD>C_VLS_DOWN_GRD_DYN,rapid intervention by the control system is initiated. This interventionis also initiated if the post-cat sensor signal were to fall directlyfrom 46 to 50. In FIG. 3 the constant C_VLS_DOWN_GRD_DYN is plotted asthe interval 52 relative to the signal value 46. The gradient resultingfrom the values 46 and 50 is shown as interval 54. The rapid fall in thepost-cat sensor signals illustrated in FIG. 3 necessitates rapidintervention in mixture formation. This intervention is initiated in theconventional manner. FIG. 3 likewise shows that the rising post-catsignal 56 has the direct result of resetting the dynamic state 58.

In the example shown in FIG. 3 the post-cat signal 56 rises aftercontrol intervention has taken place so that regular operation is thenresumed due to the reset dynamic mode 58.

Not shown in the Figures is the fact that the constantsC_CTR_VLS_DOWN_CONST, C_CTR_VLS_DYN_THD, C_FAC_VLS_DOWN_DYN andC_VLS_DOWN_GRD_DYN may depend on other physical and chemical variables.These variables can be determined directly or with the aid of modeling.For example, the operating point dependent exhaust gas composition canbe used as the basis for calculating these constants.

As a result of the method described, individual bit changes in thepost-cat sensor signal are evaluated differently in the case of a binarypost-cat sensor. A VLS_DOWN_SIGNAL which is slowly falling or risingagain in between is not deemed to be “dynamic”. It does not necessitateany control intervention. If the signal falls somewhat more quickly,intervention takes place, preferably as a function of the operatingpoint dependent positions of the absolute value of the post-cat sensorsignal. If the signal falls very quickly, intervention takes placeimmediately. The controller speed is therefore dependent on theoperating point of the engine, in particular the mass air flow (MAF) andthe engine speed (N), and the state or operating point (VLS_DOWN) of thecatalytic converter.

In the above examples, the counter CTR_VLS_DOWN_DYN was based on asegment-synchronous calculation. However, it is also conceivable for atime-synchronous calculation to used as the basis or to relate to theoxygen mass balancing. Another option is to relate the threshold to anexhaust gas quantity. It is alternatively possible to assign the actuallambda value from the pre-cat signal to a quantity of oxygen or otherexhaust gas component and use this as a reference for the constants.

1.-11. (canceled)
 12. A Method for controlling a fuel mixture providedfor an internal combustion engine generating a flue gas stream bycombusting the mixture, the engine comprising: a catalytic converterarranged in the flue gas stream; and a lambda probe arranged downstreamof the converter relative to the fuel gas stream for providing post-catsensor signals, the method comprising: detecting a post-cat sensorsignal by the lambda probe; acquiring at least one subsequent post-catsensor signal chronologically succeeding the post-cat sensor signal bythe lambda probe; generating a comparative value by comparing thedetected subsequent post-cat sensor signal to the acquired post-catsensor signal; changing the mixture if the comparative value is greaterthan or equal to a predetermined value; and generating a number ofchronologically succeeding comparative values by repeating the detectingand the acquiring and checking a trend of the succeeding comparativevalues.
 13. The method according to claim 12, wherein the comparativevalue is a difference value between the post-cat sensor signal and thesubsequent post-cat sensor signal.
 14. The method according to claim 12,wherein checking the trend comprises calculating a calculated valuebased on the post-cat and the subsequent post cat senor signals.
 15. Themethod according to claim 14, wherein the methode further comprises afurther changing the mixture if the number of succeeding comparativevalues exeeds a minimum number and the calculated value is less than thepredetermined value.
 16. The method according to claim 14, wherein thecalculating includes defining a minimum and a maximum post-cat sensorsignals and dividing a difference value between the detected post-catsensor signal and the minimum post-cat sensor signal by a differencevalue between the maximum post-cat sensor signal and the minimumpost-cat sensor signal.
 17. The method according to claim 12, whereinchanging the mixture includes changing a frequency or an amplitude of aforced activation of the catalytic converter.
 18. The method accordingto claim 12, the flue gas stream comprising segments having rich andlean exhaust gas packets, wherein changing the mixture includes asuppression of the lean exhaust gas packets.
 19. The method according toclaim 16, wherein defining the minimum and the maximum post-cat sensorsignals is based on a current mass air flow or an engine speed.
 20. Themethod according to claim 12, wherein checking the trend includescomparing the subsequent post-cat sensor signals used for generating thenumber of the succeeding comparative values.
 21. The method according toclaim 20, wherein checking the trend is stopped if the comparedsubsequent post-cat sensor signals substantially equal.
 22. The methodaccording to claim 12, wherein the predetermined value or the number ofsucceeding comparative values is based on at least one operating pointof the engine.
 23. The method according to claim 22, wherein theoperating point is based on a current exhaust gas composition of theflue gas stream.
 24. The method according to claim 15, wherein theminimum number is based on at least one operating point of the engine.25. The method according to claim 24, wherein the operating point isbased on a current exhaust gas composition of the flue gas stream. 26.The method according to claim 12, wherein generating the number ofsucceeding comparative values or checking the trend is executed during apredetermined period.
 27. The method according to claim 12, whereingenerating the number of succeeding comparative values or checking thetrend is executed during a period based on the segments of the flue gasstream.
 28. The method according to claim 12, wherein generating thenumber of succeeding comparative values or checking the trend is basedon an oxygen mass balance of the combustion.
 29. A control unit forcontrolling a fuel mixture provided for an internal combustion enginegenerating a flue gas stream by combusting the mixture, the enginecomprising: a catalytic converter arranged in the flue gas stream; and alambda probe arranged downstream of the converter relative to the fuelgas stream for providing post-cat sensor signals, the controllingcomprising: detecting a post-cat sensor signal by the lambda probe;acquiring at least one subsequent post-cat sensor signal chronologicallysucceeding the post-cat sensor signal by the lambda probe; generating acomparative value by comparing the detected subsequent post-cat sensorsignal to the acquired post-cat sensor signal; changing the mixture ifthe comparative value is greater than or equal to a predetermined value;and generating a number of chronologically succeeding comparative valuesby repeating the detecting and the acquiring and checking a trend of thesucceeding comparative values, the control unit is operativelyconnectable to the lambda probe for detecting the post-cat sensor signaland acquiring the subsequent post-cat sensor signal.
 30. The controlunit according to claim 29, the control unit is adapted such thedetecting and the acquiring is executable by reading the post-cat andthe subsequent post-cat sensor signals.
 31. The control unit accordingto claim 29, the control unit is configured for generating thecomparative value, changing the mixture, generating the number ofsucceeding comparative values or checking the trend.